Structural fabric of the Central Metasedimentary Belt of southern Ontario, Canada, from deep seismic profiling

The Central Metasedimentary Belt boundary tectonic zone (CMBbtz) is a 10-20-km-wide zone of intense structural deformation within the 1.3-1.0 Ga Grenville orogen of southeastern Canada. The crustal structure of the exposed CMBbtz has been well studied, but its sub-Phanerozoic location and geometry beneath the urban development and nuclear stations of the Toronto region are not well known. A new 75-km Lithoprobe reflection profile acquired close to Toronto provides a clear image of the CMBbtz as a panel of southeast-dipping reflections that extends with moderate dip (<25degrees) to mid-crustal depth (similar to25 km). These dipping reflections truncate and (or) overprint a subhorizontal band of reflectivity at similar to21 km depth. The seismic line is oblique to the major structural trends; cross-dip analysis shows that the southeast-dipping reflections have a strike and dip of similar toN13degreesE and 25degrees, whereas the "subhorizontal" reflections strike and dip at similar toN65E and 20degrees, respectively. Both of these bands of reflectivity can be correlated to magnetic anomalies in the CMBbtz or its immediate footwall. Magnetic anomalies with similar strike directions are well expressed within a distinct rhomboid-shaped region (106 x 109 km) in the subsurface of western Lake Ontario, herein named Mississauga domain. Taken together, the seismic and magnetic data are inconsistent with existing models, in which the CMBbtz is extrapolated beneath Lake Ontario along a linear magnetic anomaly. We propose a revised subsurface trace of the CMBbtz along the western edge of the Mississauga domain. Small earthquakes in western Lake Ontario appear to cluster along trends co-linear with ENE magnetic anomalies, suggesting a possible degree of basement tectonic control on local intraplate seismicity. Crown Copyright (C) 2004 Published by Elsevier B.V. All rights reserved.